After a bit of Googling, I found that I could make this go away by modifying the mkspecs/defs.pro file in the directory where I had unpacked the source tarball. At the top of the file, you will see these lines:

I then re-ran the build script and everything worked with no further errors. Obviously, with a piece of software as complex as QCAD, there's no guarantee that this change won't break some feature or other, but at least having it compile and run without problems is a good start.